1. Field of the Invention
The present invention is directed to an optical character generator for an electrographic printer or copier device.
2. Description of the Related Art
Optical character generators have the job of converting print information which is present in the form of electronic data into an optical image with which a photoconductive layer, for example of a photoconductive drum, is then exposed. Subsequently, the exposed image is developed in a known way and, for example, transfer-printed onto paper.
Optical generators that are constructed as a line-like arrangement are advantageous since these work without mechanical movement. Given this type of character generation, an individual, separate light source must be present for every point that is to be image within a line. The number of light sources is thereby very high for example, it may be several 1000). Assuming identical inking of the photoconductive layer all light sources must generally exposed with the same optical energy, so that an optimum quality of the print image is achieved as a result.
The U.S. Pat. No. 4,734,714 discloses an optical character generator with an LED diode array for reproducing black-and-white image information for which a uniform illumination at every picture element is required. The LED arrays are successively arranged in a row on a substrate such that they form a printing line in which a light source is present for each reproducible pixel. Two control modules whose terminals are conducted to the terminals of the LED arrays via interconnects on the substrate are allocated to each LED array. A connection between the interconnect and LED array ensues by bonding. Whereas the one control module is responsible for the drive of the odd-numbered light-emitting diodes, the other control module is responsible for the drive of the even-numbered light-emitting diodes. The control modules contain shift registers into which the 1-bit wide image information can be serially clocked. The output of the shift register of a first control module is connected to the input of a shift register of a second control module, so that data that are written into the first control module can be forwarded to the second as well as to other, following control modules. In order to lower the clock rate upon insertion of the data into the shift register, it is proposed that individual groups of control modules be formed whose shift registers are coupled in the above-described way. In order to set the emitted optical energy of every individual light-emitting diode such that the variation of the emitted optical power of neighboring light-emitting diodes is limited, it is proposed that current-limiting means in the form of resistors or transistors be added to the individual driver components of the control modules.
The image information comprises 1 bit per pixel in the known character generator. More recent applications, however, require character generators that are suitable for the reproduction of gray-scale image information or color image information. Such image information requires pixel that each comprise a plurality of bits. Accordingly, a plurality of bits must be made available in parallel for driving of a single light source.
The European Patent Document EP-A-0 504 575 likewise discloses a character generator with an LED diode array. Due to its type, however, this character generator is hardly suitable for simultaneously representing gray scale information made up of a plurality of bits.